Arrays of ejector valves are used in sorting apparatus in which material to be sorted is directed in a product stream following an aerial path, and where certain material is removed from the stream by pulses from the ejector valves. The ejector valves control the delivery of a fluid, such as air, from a pressurised source to an ejector nozzle directed at a particular section of the product stream. A mechanism, typically comprising an electromagnet, acts to open each valve in the array selectively in response to a control signal to deliver a pulse of pressurised fluid to, and thus from, the respective nozzle thereby ejecting or removing certain material or particles from the product stream.
In one known sorting application, a sorting apparatus having pneumatic ejectors sorts particulate material according to its ability to reflect light as described in GB-A-2025038, in which detectors are responsive to light reflected from the particles of product and generate signals indicative of different qualities of the product. These signals are compared and analysed, to generate an electrical control signal, which can activate an ejector valve to remove the relevant particle from the product stream.
The relative performance of such ejector valves can be a problem in that different valves, receiving the same control signal, may perform differently due to age, machining or manufacturing variations and tolerances.
U.S. Pat. No. 4,974,622 describes a method of self compensation for duty cycle control of a solenoid valve typically used in a fuel injection engine. The method disclosed involves measuring the nominal stop time for a valve to move from a first position (start) to a second position (stop), and subsequently adjusting this stop time by a deviation time determined by actual sensing of the movement of the valve.
U.S. Pat. No. 6,889,121 describes a method to adaptively control and derive the control voltage of solenoid operated valves based on the valve closure point. An initial estimate of the valve closure point is derived and this is updated by measuring the coil current feedback in use.
The methods described above generally relate to controlling a single valve without compensating for local environmental effects due in part to a large number of valves arranged in an array as found in sorting machines.
In particular, an array of ejector valves necessarily has many ejector valves and solenoids in close proximity or adjacent to each other. A problem exists in such environments due to temperature and especially electromagnetic noise variations which can interfere or influence neighbouring valves. Therefore, problems in performance in an array of ejector valves are compounded, since the ejector valves influence each other to varying degrees, in addition to the increased temperature and manifold pressure variations and each valves individual wear and tear. It will be appreciated that, in such a complex system, many additional factors cause problems leading to overall degradation or non-conformal performance.
Hence the calibration and subsequent controlling or compensation of such an array is not a trivial exercise in such environments.
Additionally, the replacement of individual ejector valve assemblies within the array can, over time, lead to an array with a mixture of old and newer ejector valves. This may provide slightly different individual valve performances and therefore exacerbate or add to the other factors mentioned above in relation to the hostile operating environment. This can lead to differing fluid flow temporal responses for each ejector, which results in non-conformal product stream sorting or rejection.
There is therefore a desire for improved prediction and real time adjustment techniques for driving an array of ejector valves in, for example, a sorting machine, to correlate fluid flows from the array as a whole within performance targets.